Dual stack optical data storage medium and use of such medium

ABSTRACT

A dual stack optical data storage medium ( 30 ) for recording and reading by means of a focused radiation beam ( 29 ) is described. The beam enters the medium ( 30 ) through a first radiation beam entrance face ( 21 ). The medium has at least a first substrate ( 1   a ) with on at least one side of the first substrate ( 1   a ) a first layer stack ( 2 ), comprising a first information layer ( 3 ), a second layer stack ( 5 ), comprising a second information layer. The second layer stack ( 5 ) is present at a position more remote from the first radiation beam entrance face ( 21 ) than the first layer stack ( 2 ). A first transparent spacer layer ( 4 ) is present between the first layer stack ( 2 ) and the second layer stack ( 5 ). The first information layer ( 3 ) is a read only type layer or an organic write once type layer, and the second layer stack ( 5 ) consists of maximally three adjacent layers of an inorganic metallic material. In this way an optical data storage medium ( 30 ) is achieved which is compatible with a dual stack ROM version of said medium and which has a simple second layer stack ( 5 ).

The invention relates to a dual stack optical data storage medium forrecording and reading by means of a focused radiation beam entering themedium through a first radiation beam entrance face, said medium havingat least a first substrate with on at least one side of the firstsubstrate:

a first layer stack, comprising a first information layer,

a second layer stack, comprising a second information layer, said secondlayer stack being present at a position more remote from the firstradiation beam entrance face than the first layer stack,

a first transparent spacer layer between the first layer stack and thesecond layer stack.

The invention further relates to the use of such a dual stack opticaldata storage medium.

An embodiment of an optical recording medium as described in the openingparagraph is known from Japanese Patent Application JP-11066622.

Digital Versatile Disc read only (DVD-ROM) has proven to be a verysuccessful optical storage medium. The DVD-ROM standard specificationdescribes both a single-stack disk (type A; data capacity=4.7 GB) aswell as a dual-stack disk (type C; data capacity=8.5 GB). A write onceand/or rewritable medium, which is compatible with the type A and type CDVD-ROM standard, is highly desirable. Furthermore, a double-sidedversion of the single-stack disk (type B; data capacity=9.4 GB) and adouble-sided version of the dual-stack disk (type D; data capacity=17.0GB) are described. Recordable and/or rewritable media, which arecompatible with the DVD-ROM standard, are highly desirable. Recently anew format has been introduced called Blu-ray Disc (BD) with even ahigher storage capacity. This system uses a blue radiation beamwavelength and a relatively high numerical aperture of the focusedradiation beam. For this format also write once (R) and rewritable (RW)versions will be introduced.

For the single-stack DVD (type A) a compatible recordable format (DVD+R)and a nearly compatible rewritable format (DVD+RW) have been defined.For the dual-stack DVD (type C), a compatible dual-stack recordableDVD(+R) medium based on dye materials is described in non-prepublishedEuropean Patent Application no. 02075226.7 (PHNL020086) filed by thepresent applicant. A dual-stack rewritable DVD(+RW) medium is alsofeasible, but it seems that such a medium cannot be made compatible withthe DVD-ROM standard, because of the limited reflection and transmissionof the rewritable materials, e.g. phase-change materials, that are used.The increase in data capacity of a dual-stack DVD+R compared to asingle-layer DVD+R and its compatibility with read only standards areclear advantages. For the second layer stack, sufficient reflection andsufficient modulation are required for compatibility reasons. Theserequirements can be met using organic dye materials. However producing asecond layer stack based on dye materials involves at least threedeposition steps: 1) deposition of a (metallic) mirror, e.g. bysputtering; 2) deposition of a dye layer, e.g. by spincoating orevaporation; 3) deposition of a protective capping layer, e.g. byspincoating or sputtering. The use of an inorganic phase-change stackwith dielectric interference layers, as commonly used in rewritablemedia such as DVD+RW, instead of a dye-based recordable stack is notvery attractive from a cost point of view, due to the fact that itinvolves sputtering of at least four layers. Furthermore, meeting boththe reflection and modulation specifications using this kind ofinorganic recordable stack still remains an issue.

It is an object of the invention to provide a dual stack optical datastorage medium of the type mentioned in the opening paragraph which iscompatible with a dual stack ROM version of said medium and which has asimple second layer stack.

This object is achieved by a dual stack optical data storage mediumaccording to the invention which is characterized in that the firstinformation layer is one selected from the group of types consisting ofa read only layer and an organic write once layer, and that the secondlayer stack consists of maximally three adjacent layers of an inorganicmetallic material. Since no transmission requirement for the secondlayer stack exists, materials other than organic dyes may be used tomeet the reflection and modulation requirements of the second stack. Ofcourse, a stack based on such materials must be equally or moreattractive than a stack based on organic dye materials from a productionand/or cost and/or reliability point of view. The deposition of metal oralloy layers is a technique frequently applied and can generally beperformed very efficiently in a sputtering apparatus dedicated to thesubsequent deposition of layers of metals or alloys of metals. Atrilayer may be deposited using a single sputtering apparatus. No shiftfrom dry processing to wet processing is necessary for the deposition ofthe second stack. This is a clear advantage over the use of an organicdye layer, which cannot be deposited by a sputtering process. Theapplicant has recognized that a second layer stack design of the mediumis possible using a stack of maximally three inorganic metallic layerswhich meets the requirements for such a stack in order to be compatiblewith a dual layer ROM version of said medium.

Applicant has further recognized that the problem with a dual-stackrecordable, i.e. write once, medium is that prerecorded read only data,which a manufacturer would desire to put on such a medium, have to bewritten sequentially medium by medium. This problem is solved byproviding a medium wherein one of the first and second informationlayers is a read only layer with preembossed information as, e.g., in anormal DVD-ROM. The applicant have further recognized that with acombination of different types of recording layers according to theinvention it still is possible to achieve a dual stack medium compatiblewith e.g. the dual layer (=dual stack) DVD ROM standard. E.g. animportant parameter of the type C DVD-ROM standard is the reflectivityof the storage layers, which must be between 18% and 30% for each of thetwo layers. Consequently, the first information layer closest to theradiation beam entrance face of a compatible dual stack DVD+R mediumshould have a high transmission, sufficient reflection and lowabsorption. These criteria can be met for a write once layer based one.g. dye materials or a read only layer, but cannot be met for ametallic write once layer, which has a relatively high opticalabsorption. In other words, the first information layer is a write once,e.g. a dye based layer with a relatively low absorption, layer or readonly ROM layer (low absorption), while the second layer stack is a stackaccording to the invention. It should be noted that for the firstinformation layer any type of organic layer with a relatively lowabsorption at the radiation beam wavelength is suitable and that thereis no restriction to dye layers specifically.

In an embodiment the second layer stack is a bilayer of a phase changealloy layer and a metallic reflective layer, the metallic reflectivelayer being present closer to the radiation beam entrance face than thephase change alloy layer. The inorganic second layer stack may consistof two sputtered layers, e.g. a phase-change layer with a metallic layeron top to enhance the reflectivity.

In an embodiment the metallic reflective layer mainly comprises Al. Forthe metallic layer, aluminum (Al) is of particular interest since itforms a very thin, closed and chemically inert aluminumoxide layer uponexposure to air or an oxygen environment, before bonding the secondlayer stack to the first layer stack. The specific properties of such analuminumoxide layer eliminate the need to deposit an additionalprotective capping layer.

Preferably the phase change layer is an alloy comprising Sb and Te. Aphase change layer of such a composition is relatively easy to deposit.A preferred composition is Sb₂Te₃, a stable compound of which materialthe properties are well known. The physical recording mechanisms of saidinorganic stacks may include mark formation by alloying and/orphase-changing and/or sintering and/or segregation and/or bubbleformation and/or ablation and/or hole formation at elevated temperaturesduring writing.

Preferably the thickness of the Al layer is selected from the range of5-10 nm and the thickness of the phase change layer is selected from therange of 10-40 nm. Layer thicknesses in these ranges give optimalperformance as far as writability, sensitivity and reflectivity.Writability e.g. concerns the modulation depth of written marks, i.e.the normalized difference in reflection between an unwritten portion ofthe stack and a written mark in the stack when read out by the spot ofthe focused radiation beam.

In a special embodiment the medium further comprises a second radiationbeam entrance face opposite from the first radiation beam entrance faceand

a third layer stack, comprising a third information layer selected fromthe group consisting of a read only layer and an organic write oncelayer,

a fourth layer stack, being present at a position more remote from thesecond radiation beam entrance face than the third layer stack, saidfourth layer stack consisting of maximally three adjacent layers of aninorganic metallic material, and

a second transparent spacer layer between the third layer stack and thefourth layer stack. The maximum data capacity of a single-sideddual-stack optical data storage medium is limited, e.g. to 8.5 GB forDVD. In order to store two versions of a movie in DVD format, includingextra features, on one disc, e.g. a full-screen and wide-screen versionas is commonly done for movies distributed in the U.S., 8.5 GB ofstorage capacity is generally insufficient. Therefore a compatibledouble-sided dual-stack optical recording medium is proposed. Theproposed medium is compatible with its read only version, e.g. the typeD DVD-ROM standard and consequently has a doubled total storagecapacity, e.g. 17.0 GB in case of the type D DVD-ROM.

For an optical data storage medium compatible with the dual stackDVD-ROM specification the effective reflection level of the stacks is atleast 0.18 at a radiation beam wavelength of approximately 655 nm.

For an optical data storage medium compatible with the dual stack BDspecification the effective reflection level of the stacks ranges from0.04 to 0.08 for dual-layer BD-RW and 0.12 to 0.24 for single-layerBD-RW, at a radiation beam wavelength of approximately 405 nm.

The invention will be elucidated in greater detail with reference to theaccompanying drawings, in which

FIG. 1 shows a schematic layout of an embodiment of a dual-stack opticaldata storage medium according to the invention,

FIG. 2 shows a schematic layout of an embodiment of a double sideddual-stack optical data storage medium according to the invention,

In FIG. 1 a dual stack optical data storage medium 30 for recording andreading by means of a focused radiation beam 29 is shown. The radiationbeam 29 enters the medium 30 through a first radiation beam entranceface 21. The medium has at least a first substrate 1 a with on at leastone side of the first substrate 1 a a first layer stack 2, comprising afirst information layer 3, a second layer stack 5, being present at aposition more remote from the first radiation beam entrance face 21 thanthe first layer stack 2. A first transparent spacer layer 4 is presentbetween the first layer stack 2 and the second layer stack 5. The firstinformation layer 3 is a read only layer or an organic write once layerand the second layer stack 5 consists of maximally three adjacent layersof an inorganic metallic material. The embodiment will now be discussedin more detail.

Substrate 1 a has a servo pregroove or guide groove pattern in itssurface at the side of the first layer stack 2 and is made ofpolycarbonate (n=1.58) and has a thickness of 580 μm. The servopregroove is used for guiding the focused radiation beam 29 duringrecording and/or read out. First layer stack 2 is a write once stackcomprising a first information layer 3 made of a cyanine dye or azo dye(n=2.2; k=0.01) having a thickness of 90 nm. The dye may be deposited byspincoating or evaporation. A semi transparent reflective layer of Au(n=0.28; k=3.9) having a thickness of 8 nm is present between the firstinformation layer 3 and the spacer layer 4 and deposited by e.g.sputtering. The first transparent spacer layer 4 is made of anUV-curable resin or a pressure-sensitive adhesive (PSA) (n=1.5) with athickness of 40-60 μm. The second layer stack 5 is a bilayer of a phasechange alloy layer 6 b of Sb₂Te₃ (n=2.9; k=4.8) having a thickness of 12nm and a metallic reflective layer 6 a of Al (n=1.97; k=7.83) having athickness of 6 nm. The metallic reflective Al layer 6 a is presentcloser to the radiation beam entrance face 21 than the phase changealloy Sb₂Te₃ layer 6 b. A second substrate 1 b, made of polycarbonate(n=1.58) and having a thickness of 580 μm, is present adjacent thesecond layer stack 5. The substrate 1 b has a servo pregroove or guidegroove pattern in its surface at the side of the second layer stack 5.

The listed optical parameters n and k are for λ=655 nm which is theradiation beam wavelength. The calculated reflection and transmissionare:

First Layer Stack 2:

-   Reflection (R1)=20%-   Transmission (T1)=64%-   Effective reflection from first layer stack 2:=R1=20%    Second Layer Stack 5:-   Reflection (R2)=62%-   Effective reflection from second layer stack 5:=T1×T1×R2=25%    The effective reflection of both layers is in full compliance with    the DVD-ROM standard: 18%<R<30%.    In a variant of this embodiment the first layer stack comprises a    read only information layer. Such an information layer may e.g. be a    11 nm Au layer deposited in the pregroove pattern. In this case    R1=22%, T1=61%, R2=62% and the effective reflection from second    layer stack 5:=T1×T1×R2=23%.

Note that the substrate 1 a may be replaced by a relatively thin, e.g.100 μm, cover layer of a spincoated and cured UV curable material or asheet of plastic with a pressure sensitive adhesive (PSA). Such a coverlayer is e.g. used for the high density BD version of the optical datastorage medium.

In FIG. 2 a double-sided dual stack optical data storage medium 30 isshown compatible with the type D DVD-ROM standard. Reference numerals 1a, 21, 2, 3, 5, 6 a, 6 b correspond to the description of FIG. 1. Thefirst transparent spacer layer 4 is made of an UV-curable resin and hasa servo pregroove or guide groove pattern in its surface at the side ofthe second layer stack 5. Substrate 1 b of FIG. 1 is replaced by acoupling layer 12. The medium further comprises a second radiation beamentrance face 22 opposite from the first radiation beam entrance face 21for recording and reading in a third layer stack 7, comprising a thirdinformation layer 8 selected from the group consisting of a read onlylayer and an organic write once layer, and a fourth layer stack 10,being present at a position more remote from the second radiation beamentrance face than the third layer stack 7. The fourth layer stack 10consists of maximally three adjacent layers of an inorganic metallicmaterial.

A second transparent spacer layer 9 is present between the third layerstack 7 and the fourth layer stack 10. The layers and stacks 1 b, 7, 8,9, 10, 11 a and 11 b are identical to respectively the layers and stacks1 a, 2, 3, 4, 5, 6 a, 6 b. Hence a dual sided dual stack medium isprovided with identical design on both sides bonded together by couplinglayer 12 which may be a PSA with a thickness of 20-300 μm. Depending onthe thickness of the substrates 1 a and 1 b and the spacer layers 4 and9, the thickness of the coupling layer 12 may be adjusted in order tohave the total thickness of the medium 30 not exceed the maximumthickness as specified in the DVD disk standard, i.e. 1500 μm. Thethickness range of the substrate however is also limited in order toprevent occurrence of excessive optical aberrations in the focusedradiation beam 29 used for reading and writing in the informationlayers.

The pregroove (or guide groove) of the second layer stack 5 and thefourth layer stack 10 may also be present in the coupling layer 12 inwhich case the coupling layer may constitute a sheet of plastic withpregrooves on both sides. In this case, spacer layers 4 and 9 mayconstitute an UV-curable resin or pressure-sensitive adhesive (PSA)without pregroove.

A dual sided BD version is also possible, in which case two cover layersare present at the position of substrate 1 a and 1 b of FIG. 2 and atleast one substrate, e.g. 1 a or 1 a/1 b, is present between the secondand fourth layer stacks 5 and 10 instead of the coupling layer 12.

It should be noted that the above-mentioned embodiment illustratesrather than limits the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

According to the invention a dual stack optical data storage medium forrecording and reading by means of a focused radiation beam is described.The beam enters the medium through a first radiation beam entrance face.The medium has at least a first substrate with on at least one side ofthe first substrate a first layer stack, comprising a first informationlayer, a second layer stack, comprising a second information layer. Thesecond layer stack is present at a position more remote from the firstradiation beam entrance face than the first layer stack. A firsttransparent spacer layer is present between the first layer stack andthe second layer stack. The first information layer is a read only typelayer or an organic write once type layer, and the second layer stackconsists of maximally three adjacent layers of an inorganic metallicmaterial. In this way an optical data storage medium is achieved whichis compatible with a dual stack ROM version of said medium and which hasa simple second layer stack.

1. A dual stack optical data storage medium for recording and reading bymeans of a focused radiation beam entering the medium through a firstradiation beam entrance face, said medium having at least a firstsubstrate with on at least one side of the first substrate: a firstlayer stack, comprising a first information layer, a second layer stack,comprising a second information layer, said second layer stack beingpresent at a position more remote from the first radiation beam entranceface than the first layer stack, a first transparent spacer layerbetween the first layer stack and the second layer stack, characterizedin that the first information layer is one selected from the group oftypes consisting of a read only layer and an organic write once layer,and that the second layer stack consists of maximally three adjacentlayers of an inorganic metallic material.
 2. A dual stack optical datastorage medium as claimed in claim 1, wherein the second layer stack isa bilayer of a phase change alloy layer and a metallic reflective layer,the metallic reflective layer being present closer to the radiation beamentrance face than the phase change alloy layer.
 3. A dual stack opticaldata storage medium as claimed in claim 1, wherein the metallicreflective layer mainly comprises Al.
 4. A dual stack optical datastorage medium as claimed in claim 1, wherein the phase change layer isan alloy comprising Sb and Te.
 5. A dual stack optical data storagemedium as claimed in claim 3, wherein the thickness of the Al layer isselected from the range of 5-10 nm and thickness of the phase changelayer is selected from the range of 10-40 nm.
 6. A dual stack opticaldata storage medium as claimed in claim 1, wherein the medium furthercomprises a second radiation beam entrance face opposite from the firstradiation beam entrance face and a third layer stack, comprising a thirdinformation layer selected from the group consisting of a read onlylayer and an organic write once layer, a fourth layer stack, beingpresent at a position more remote from the second radiation beamentrance face than the third layer stack, said fourth layer stackconsisting of maximally three adjacent layers of an inorganic metallicmaterial, and a second transparent spacer layer between the third layerstack and the fourth layer stack.
 7. A dual stack optical data storagemedium as claimed in claim 1, wherein the effective reflection level ofthe stacks is at least 0.18 at a radiation beam wavelength ofapproximately 655 nm.
 8. Use of an dual stack optical data storagemedium as claimed in claim 1, in an optical data storage device suitablefor reading a dual stack ROM version of said medium.